Except your client does not live in a bubble. It needs to communicate and send emails to other clients.
In order to combat spam, many popular email providers rank messages using an array of criteria, coming from a known and well publicised email provider is one such criteria and not coming from a well known provider is one such criteria that redirects your email to many people's spam folders.
So the question remains, how do we ensure that nostr relays don't start to use exclusionary metrics to avoid spam and abuse?
Nostr has many issues with the decentealizatuon concept. And if we are honest Nostr is already some kind of centralized as we have not thousands of relays. You can solve the issue with spam with paid relays but then the whole concept is senseless
Yeah, I think relays need an incentive model to share data with one-another. That way if you want to switch to another relay, you can without worrying about data loss.
Assuming you are paying for spam filtering and content quality and not for "hosting".
And here comes the big problem with privacy into the game